Fuel cells consist essentially of two electrodes that are in contact with an electrolyte. For example, the electrolyte can be a water solution of an acid, such as phosphoric acid, or, as explained below, it can be a solid material such as a permeable metal oxide through which ions can migrate. In the case of a liquid electrolyte solution, two porous electrodes are immersed within it; through these electrodes, such reactants as hydrogen and oxygen are conveyed into contact with the electrolyte. The hydrogen and oxygen react to release ions and electrons, and water is produced. The electrons do useful work in an external circuit, whereas the ions flow through the electrolyte from one electrode to the other to complete the internal circuit in the cell.
Fuel cell technology is relatively old and well understood. (See, for example, a publication by NASA entitled, “Fuel Cells—A Survey”, NASA SP-5115 published in 1973.) Every fuel cell consists of an electrolyte material which is sandwiched between two porous electrodes, an anode and a cathode. The input fuel passes through the anode wherein it is split into ions and electrons. The electrons are conducted through an external circuit while the ions move through the electrolyte toward the oppositely charged cathode. At the cathode, the ions combine with oxygen to form water and, depending on the fuel, carbon dioxide.
In most liquid-electrolyte fuel cells, platinum coats both the anode and cathode; the side of the electrode that is adjacent to the electrolyte serves as a catalyst for the oxidation and reduction processes. Fuel and oxidant gases are supplied to the back of the anode and the cathode respectively, and both the anode and cathode are electrically conductive.
Solid-oxide fuel cells employ a thin solid metal-oxide electrolyte through which oxygen ions can diffuse. A porous cathode electrode and a porous anode electrode are created during the fabrication process on opposite sides of the electrolyte.
Solid-state electrolytes can withstand the higher operating temperatures that correspond to greater specific power production from fuel cells. Fuel cells that use solid electrolytes are called ceramic fuel cells or, more specifically, solid-oxide fuel cells, because the electrolyte is a thin layer of solid metal oxide.
The majority of solid oxide fuel cell (SOFC) developers are pursuing a planar cell geometry with an anode supported cell design (ASC) with metal interconnects. The major challenges of the ASC technology relates to fabrication and reliability, particularly in regards to stacks of cells. A thin metal oxide electrolyte (such as yttria stabilized zirconia, or YSZ), on the order of 10-15 microns (um) thick, is supported on a thick cermet anode (500 to 1,000 um thick) composed of nickel oxide and yttria stabilized zirconia (NiO-YSZ). The anode/electrolyte bi-layer is sintered as a unit, followed by application of a thin cathode, usually 25-50 um thick, which is then fired at a lower temperature.
There are a number of problems for the ASC cell which include: 1) shrinkage matching of the thick NiO-YSZ cermet and the thin YSZ electrolyte, which has been a critical and challenging problem; 2) as the NiO in the anode is reduced to nickel metal there is a volume change that can generate stresses within the anode and cause fracture and failure of the thin YSZ electrolyte; 3) the anode is sensitive to leaks of oxygen which can cause oxidation of the Ni metal to Ni-oxide resulting in a sudden expansion of the anode and failure of the cell (as can happen at two critical times; first during operation of the cell, if there are pin-hole leaks in the electrolyte then air can leak through, leading to a localized chemical expansion as Ni-metal is oxidized, a crack starts to grow and then it causes failure of the cell, in hours or days—second, on cooling down the stack, the anode should be kept in a reducing environment so the Ni-metal does not oxidize, which is a challenge to developers where the fuel cell might be operating on reformed natural gas, jet fuel, gasoline, etc.; a reducing gas should be used to protect the fuel electrode as the stack is cooled; a brief mistake on cooling can cause the entire stack to fail all at once, resulting in the failure of hundreds of cells); 4) to provide enough strength, the rather weak Ni-YSZ ASC anodes must be made thick, which can lead to diffusion problems in the anode (which works against achieving high fuel utilization rates that are required for commercial applications); 5) the cells are fragile and can not tolerate the high compressive loading that is required for the compression type seals that are used with the ASC stacking technology (which has required some manufacturers to install additional metal sealing plates, called cassettes, which add to overall complexity and general materials challenges; 6) the anode, cathode, and electrolyte layers must be fired simultaneously up to 1,250 C so as to bond the cathode to the electrolyte, a temperature approximately 400 C higher than the fuel cell's operating temperature, which can result in significant chemical reactivity during fabrication, thus limiting potentially better performing cathode compositions.